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Backplane
A backplane (or "backplane system") is a group of connectors connected in parallel with each other, so that each pin of each connector is linked to the same relative pin of all the other connectors forming a computer bus. It is used as a backbone to connect several printed circuit boards together to make up a complete computer system. Backplanes commonly use a printed circuit board but wire wraped backplanes have also been used in minicomputers and high reliability applications. Early personal computers like the Apple II and the IBM PC integrated an internal backplane for expansion cards. While a motherboard may include a backplane for the addition of feature cards, a backplane can stand alone as a separate entity. A backplane is generally differentiated from a motherboard by the lack of on-board processing and storage elements. A backplane uses plug-in cards for storage and processing. Backplanes are normally used in preference to cables because of their greater reliability. In a cabled system, the cables need to be flexed every time that a card is added to or removed from the system; and this flexing eventually causes mechanical failures. A backplane does not suffer from this problem, so its service life is limited only by the longevity of its connectors. For example, the DIN 41612 connectors used in the VMEbus system can withstand 50 to 500 insertions and removals (called mating cycles), depending on their quality. In addition, there are bus expansion cables which will extend a computer bus to an external backplane, usually located in an enclosure, to provide more or different slots than the host computer provides. These cable sets have a transmitter board located in the computer, an expansion board in the remote backplane, and a cable between the two. Active versus Passive backplanes Backplanes have grown in complexity from the simple ISA (used in the original IBM PC) or S-100 style where all the connectors were connected to a common bus. Because of limitations inherent in the PCI specification for driving slots, backplanes are now offered as passive and active though the general usage is referred to as passive even when active components are present in the bus. True passive backplanes offer no active bus driving circuitry. Any desired arbitration logic is placed on the daughter cards. Active backplanes include chips which buffer the various signals to the slots. The distinction between the two isn't always very clear, but may become an important issue if a whole system is expected to have no single point of failure. A passive backplane, even if it is single, is not usually considered a SPOF. Active backplanes are more complicated and thus have a non-zero risk of malfunction. Backplanes Versus Motherboards When a backplane is used with a plug-in single board computer (SBC) or system host board (SHB), the combination provides the same functionallity as a motherboard providing processing power, memory, I/O and slots for plug-in cards. While there are a few motherboards that offer more than 8 slots, that is the traditional limit. In addition, as technology progresses, the availability and number of a particular slot type may be limited in terms of what is currently offered by motherboard manufacturers. On the other hand, backplane architecture is somewhat unrelated to the SBC technology that is plugged into it. There are some limitations to what can be constructed in that the SBC chip set and processor have to provide the capability of supporting the slot types. In addition, virtually an unlimited number of slots can be provided with 20, including the SBC slot, as a practical though not an absolute limit. Thus, a PICMG backplane can provide any number and any mix of ISA, PCI, PCI-X, and PCI-e slots limited only by the ability of the SBC to interface to and drive those slots. For example, an SBC with the latest i7 processor could interface with a backplane providing up to 19 ISA slots to drive legacy I/O cards. Butterfly Backplanes Some backplanes are constructed with slots on both sides. These are not the same as midplane backplanes. A butterfly backplane is typically constructed to maximize the number of slots for the least vertical height. The backplane would be mounted vertically in a chassis oriented front to back and the plug-in SBC and cards would lay flat, protruding out both sides of the backplane. This, for example, allows the use of up to four full height boards in a 2U chassis. Backplanes in storage Servers commonly have a backplane to attach hot swappable hard drives, backplane pins passing directly into hard drive sockets without cables. They may have single connector to connect one disk array controller or multiple connectors that can be connected to one or more controllers in arbitrary way. Backplanes are commonly found in disk enclosures, disk arrays, and servers. Backplanes for SAS and SATA HDDs most commonly use the SGPIO protocol as means of communication between the HBA and the backplane. Alternatively SCSI Enclosure Services can be used. With Parallel SCSI subsystems, SAF-TE is used in computers, mostly in blade servers, where server blades reside on one side and the peripheral (power, networking, and other I/O) and service modules reside on the other. Midplanes are also popular in networking and telecommunications equipment where one side of the chassis accepts system processing cards and the other side of the chassis accepts network interface cards. Platforms PICMG A Single Board Computer meeting the PICMG 1.3 specification and compatible with a PICMG 1.3 backplane is referred to as a System Host Board. In the Intel Single Board Computer world, PICMG provides standards for the backplane interface: PICMG 1.0, 1.1 and 1.2PICMG 1.0, 1.1 and 1.2 provide for ISA and PCI support with 1.2 adding PCIX support. PICMG 1.3PICMG 1.3PICMG 1.3 SHB Express Resources provides for PCI-Express support. See also *Motherboard *Switched fabric *Daughterboard *M-Module References Category:Computer buses ca:Backplane de:Backplane es:Backplane fr:Fond de panier ja:バックプレーン pt:Backplane